1. Field of the Invention
This invention pertains to compositions and methods for transporting material across cell membranes, and methods for making such compositions.
2. Introduction
Cell membranes delimit the outer boundaries of cells, and regulate transport into and out of the cell interior. Made primarily of lipids and proteins, they provide a hydrophilic surface enclosing a hydrophobic interior across which materials must pass before entering a cell. Although many small, lipophilic compounds are able to cross cell membranes passively, most compounds, particles and materials must rely on active mechanisms in order to gain entry into a living cell.
Transmembrane Transport
Regulation of transport into and out of a cell is vital for its continued viability. For example, cell membranes contain ion channels, pumps, and exchangers capable of facilitating the transmembrane passage of many important substances. However, transmembrane transport is selective: in addition to facilitating the entry of desired substances into a cell, and facilitating the exit of others, a major role of a cell membrane is to prevent uncontrolled entry of substances into the cell interior. This barrier function of the cell membrane makes difficult the delivery of markers, drugs, nucleic acids, and other exogenous material into cells.
Over the last decade, peptide sequences that can readily enter a cell have been identified. For example, the Tat protein of the human immunodeficiency virus 1 (HIV-1) is able to enter cells from the extracellular environment (e.g., Fawell et al. P. N. A. S. 91:664-668 (1994)). Such uptake is reviewed in, for example, Richard et al., J. Biol. Chem. 278 (1):585-590 (2003).
Such molecules that are readily taken into cells may also be used to carry other molecules into cells along with them. Molecules that are capable of facilitating transport of substances into cells have been termed “membrane translocation signals” (MTS) as described in Tung et al., Advanced Drug Delivery Reviews 55:281-294 (2003). The most important MTS are rich in amino acids such as arginine with positively charged side chains. Molecules transported into cell by such cationic peptides may be termed “cargo” and may be reversibly or irreversibly linked to the cationic peptides. An example of a reversible linkage is found in Zhang et al., P. N. A. S. 95:9184-9189 (1994)).
MTS molecules are discussed in, for example, Wender et al., P. N. A. S. 97:13003-13008 (2000); Hällbrink et al., Biochim. Biophys. Acta 1515:101-109 (2001); Derossi et al., Trends in Cell Biology 8:84-87 (1998); Rothbard et al., J. Med. Chem. 45:3612-3618 (2002); Rothbard et al., Nature Medicine 6 (11):1253-1247 (2000); Wadia et al., Curr. Opinion Biotech. 13:52-56 (2002); Futaki et al.; Bioconj. Chem. 12:1005-1011 (2001); Rothbard et al., U.S. Pat. No. 6,306,993; Frankel et al., U.S. Pat. No. 6,316,003; Rothbard et al., U.S. Pat. No. 6,495,663; and Monahan et al., U.S. Pat. No. 6,630,351. All patents and publications, both supra and infra, are hereby incorporated by reference in their entirety.
The uptake facilitated by MTS molecules is typically without specificity, enhancing uptake into most or all cells. Thus, although MTS molecules are capable of entering cells, and may be capable of enhancing the transport of other molecules linked to MTS molecules into cells, control and regulation of such transport remains difficult. However, it would be desirable to have the ability to target the delivery of cargo to a type of cell, or to a tissue, or to a location or region within the body of an animal. Accordingly, there remains a need in the art to target, to control and to regulate the delivery of cargo molecules by MTS molecules.